Acoustic wave sensors for measuring the properties of physical, chemical and biological effects are well known. Such sensors can be categorized as oscillator based or RFID based. Both systems take advantage of the physical, chemical and biological effects perturbing acoustic wave velocity within piezoelectric material. With oscillator based acoustic wave sensors, a delta frequency (Δf) offset is obtained according to the Sauerbrey equation. RFID based acoustic wave sensors can also detect a delta frequency (Δf) as well as due to velocity change, change in time (Δt), change in frequency (Δf), change in phase (Δφ) or a change in the correlation pattern (Δc).
Difficulties with oscillator based and RFID based systems become evident when multiple measurements of physical, chemical and biological effects are desired, forcing the system to become complex. For oscillator based systems, either multiple oscillators must be used or a complex switching network provided to switch in the various acoustic wave devices as the feedback elements of a single oscillator. Attention must also be given to the transient and settling times of a single oscillator. RFID based systems have upper limits related to how many arrays can be placed on a single acoustic wave RFID device.
Oscillator based sensor detection systems utilizing acoustic wave devices as part of the feedback element present concerns with respect to their operating characteristics, especially if more than one acoustic wave device is used. A first concern involves the stability of the oscillator due to thermal drift and load pulling of the amplifier portions of the circuit. Another concern is instability due to possible coupling of modes between adjacent acoustic wave devices which may introduce injection-locking phenomena from stray coupling within the oscillators. For a typical acoustic wave sensor system, at least two or more oscillator circuits are used, one for a reference acoustic wave and the others for measuring specified effects. This can null out any temperature, physical shock, pressure or other attributes that the acoustic wave devices are subjected to. Concern with injection locking within an array of detection oscillators requires both skilled electronic and mechanical design in order to shield or separate possible influencing signals.
The following publications outline the sensitivity of oscillators subjected to external injected signals:    P. J. Edmonson et al., “Injection locking techniques for a 1 GHz digital receiver using acoustic wave devices,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 39, no. 5, pp. 631-637, September 1992.    P. J. Edmonson et al, “The application of injection-locked oscillators to wireless communication systems,” Proceedings of the 1992 Canadian Conference on Electrical and Computer Engineering, Toronto, ON, pp. MM2.2.1-MM2.2.4, 13-16 Sep. 1992.
Possibly the largest concern with an oscillator detection system is the loss of possible information with respect to detected substances due to an averaging effect of the oscillator. Hunt et al., “Time-dependent signatures of acoustic wave biosensors,” Proceedings of the IEEE, vol. 91, pp. 890-901, 2003, developed an analytical relationship from the complex reciprocity relation and time-dependent perturbation theory from the modified Sauerbrey equation.
      Δ    ⁢                  ⁢    f    =      -                            2          ⁢                      f            u            2                    ⁢                      h            f                                                              ρ              q                        ⁢                          μ              q                                          ⁡              [                  Δρ          -                      Δμ                          V              s              2                                      ]            
Where Vs is the acoustic velocity; ρ is the density of the film; hf is the thickness of the film; μq and ρq are the shear stiffness and density of the quartz crystal, respectively; μ is the stiffness of the film; Δ is the difference between perturbed and unperturbed (denoted by subscript u) quantities. The stiffness of the film, μ, is affected by the conformational change of the recognition molecules. Conformational change is the change in the molecular structure of the MRE. In electrical terms it is equivalent to the phase change of the MRE. If the detected substances vary slightly this would also cause a slight change in the molecular structure of the receptor molecule and several values of Δμ would exist, such as Δμa, Δμb, Δμc, etc. Edmonson et al., “SAW-based carrier recovery without phase ambiguity for 915 MHz BPSK wireless digital communications,” 1992 IEEE Ultrasonics Symposium Proceedings, Tucson, Ariz., pp. 241-244, 20-23 Oct. 1992 has demonstrated that, within an oscillator circuit, independent changes are averaged to produce a single frequency output.
Other concerns that this invention addresses are the problems associated with the independent nature of the several amplifier components used within multiple oscillator circuits. These amplifier components are subjected to internal thermal cycling which changes their operating characteristics and affects the frequency stability of the oscillators. To reduce overall complexity, only one amplifier component is used within one oscillator and the various acoustic wave devices are multiplexed to this amplifier component. This then introduces load pulling, since each acoustic wave device is not identically impedance matched with respect to the other acoustic wave devices, and frequency instability is introduced.
The present invention utilizes a passive multi-element structure as the basis for its detection system. Further, the invention utilizes a swept frequency response which broadens the investigative range and can detect anomalies of the physical, chemical and biological effects which would be challenging to accomplish within an oscillator based detection system but can be implemented within an interrogation type sensor system such as an RFID sensor application.